Canadian version of a restraining order comes to mind. It can be delivered to mother in law's house.
Otherwise it would depend on how I felt if I ignored her knock or opened the door and blasted her. Though I have found with my gfgbro that nothing short of telling him that I will call the police if he doesn't leave gets through. Even after telling him to NOT come here, NOT call, NOT leave things on my doorstep, and NOT send me letters he still interprets ANY and ALL interaction as a sign that I want a relationship and am willing to "forgive" him. I am at the point where I am working on that. Mostly because I finally realized that when my kids read a book he gives them or wears a shirt he gives them then they have awful nightmares and wake up in panic attacks begging him to not hurt them or me or husband. NO CHILD should have that - and he says that I cause that and NOT his behavior. I cause it by telling them that his awful behavior is awful, and not telling them it is normal. He actually told me that. And I LAUGHED.
Personally, I would likely put a note on the door that if she knocks or her family knocks you will call the police. If she tries your doorknob it will be an attempted unlawful entry and you WILL press charges as until she is clean (not on methadone) and has done the 12 steps and grown up then you will NOT have anything to do with her.
ANY interaction will be seen as you trying to mend relationships. Calling the police or getting a restraining order is different and sends a clear message. mother in law will have to cope. Alanon might be a good thing to explore this week to get some helpful ideas and support.
